Sensor array for detecting electrical characteristics of fluids

ABSTRACT

A sensor array for detecting the distribution of fluids having different electrical characteristics, comprising a multilayer structure including a first layer which defines an array of spaced apart sensor electrodes, a second layer separated from the first layer by dielectric material and defining a conductive screen, and a third layer separated from the second layer by dielectric material and defining a series of spaced apart elongate connections, each sensor being connected to a respective connection by a respective conductive path extending through an opening in the conductive screen defined by the second layer.

This application is a 371 of PCT/GB98/02271, filed on July 29, 1998.

BACKGROUND OF THE INVENTION

The present invention relates to a flow control system, and inparticular to a system for controlling the flow of a mixed phase fluidthrough a vessel. The term “mixed phase fluid” is used to cover fluidsmade up of for example suspended particulates, liquids emulsions and gasderived from different constituents for example oil and water, or liquidand gas derived from the same constituent.

Oil field production systems generally comprise separator plant in whichraw fluid pumped from an oil bearing formation is separated into itsconstituent parts, that is volatile gases, liquid petroleum products,water and particulates. The nature of the input fluid to the separatorplant can vary widely over relatively short periods of time. For examplea large proportion of the flow may be made up of water for a firstperiod of time and oil and gas for a second period of time. It isdifficult with a separator of fixed configuration to satisfactorilyprocess different flows when flow conditions change in an unpredictablemanner.

In a conventional separator, an inlet flow is generally passed through astack of inclined plates within a relatively large vessel, the inclinedplate encouraging the separation of water, oil and gas into separatesuperimposed layers. Gas can then be extracted from an upper section ofthe separator vessel and the water and oil can be separated by a simpleweir separator plate the height of which is arranged to be above theinterface between the water and oil layers. If the input flow is suchthat the separator plates become largely filled with a foam or emulsionof for example oil and water the separation performance is significantlydegraded. Similarly, if a large volume of water is delivered to theseparator in a relatively short period of time, it can be difficult tomaintain the water/oil interface below the level of the weir separator.

With such problems in mind, the normal approach to separator design hasbeen to provide a relatively large capacity separator which is capableof dealing with a wide range of conditions by in effect accepting widefluctuations in separator plate efficiency and water/oil interfacelevels. As a result of this design philosophy, separator plant can makeup a significant proportion of the size and weight of oil fieldequipment. This is a particular problem in the case of offshore oilfields where the size and weight of offshore equipment determines theeconomic viability of some oil bearing formations.

Attempts have been made to monitor separator performance in particularcircumstances so as to be able to match separator design to expectedseparator operating conditions. The equipment used has generallyrequired the mounting of heavy gamma ray sensors on separator equipment.The use of such equipment for monitoring routine operating conditions isnot appropriate.

Extensive work has been conducted to enable flow conditions within forexample circular-section pipes to be monitored. For example, U.S. Pat.No. 5,130,661 describes a capacitance sensor system in which an array ofcapacitor plates is disposed around the outer periphery of a pipethrough which a mixture of oil, water and gas is passing. By appropriatemanipulation of output signals derived from the sensors, an image can bebuilt up of the flow cross-section. Such sensing systems have been usedfor example to estimate mass flow rates of different phases but it hasnot been suggested that the output of such systems can be used toachieve real-time process control.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a fluid flow controlsystem which obviates or mitigates the forementioned problems.

According to the present invention, there is provided a system forcontrolling the flow of a mixed phase fluid through a vessel, comprisingat least one flow control device located in an Inlet to or outlet fromthe vessel, a plurality of sensors distributed about the vessel to sensecharacteristics of the flow adjacent the sensors, means for monitoringoutputs of the sensors to detect the location of a boundary betweenphases within the flow, and means for controlling the flow controldevice to maintain the boundary location within pre-determined limits.

The present invention may be applied to the control of processes such asinclined plate separation and weir plate separation. The supply of fluidto or the extraction of different phases from the vessel can becontrolled so as to maintain interfaces between different phases atdesired levels. For example, in the case of a weir plate, used toseparate water and oil, the rate at which water is removed can becontrolled to maintain the water/oil interface above a water outlet andbelow an upper edge of the weir. Similarly, the supply of fluid to aninclined plate separator can be controlled to prevent the separatorbecoming ineffective for example as the result of the build-up ofemulsions between the plates.

Preferably the sensors are embedded in vessel surfaces adjacent to whichthe fluid flows, for example in the surfaces of the plates of inclinedplate or weir plate separators. The sensors may be capacitance sensors,pressure sensors or conductivity sensors for example. In the case of aninclined plate separator, the sensors may be supported in for examplethree vertical arrays, the arrays being spaced apart in the directionsof fluid flow through the separator.

The sensors may be provided in a multilayer structure comprising a toplayer of conductance sensors, a screening layer, and a layer carrying aseries of connections, each layer being separated from the other bydielectric material, conducting paths between the sensors and theconnections passing through the screening layer. This arrangement isadvantageous because it avoids “cross-talk” between the sensors and theconnections. This makes conductance measurements more accurate, andallows some sensors to be used as sources of electric fields whilstother sensors are simultaneously used as detectors of electric fields.

The screening layer preferably comprises a layer of conducing material.Screening conducting material is preferably located between theconnections to prevent cross-talk between the connections. A mostpreferably way of providing the multi-layer structure is by bondingtogether a series of printed circuit boards.

The sensors may be supported in a vertical array, for example atvertically spaced positions on a weir separator, or at vertically spacedpositions on an elongate support extending vertically through the fluidflow. The sensors may be arranged to have a common axis or lie in acommon plane, so that measurements of electrical properties are madebetween adjacent sensors.

The elongate support may comprise a rod having two faces subtending anangle of less than 180 degrees, the faces being provided with an arrayof sensors. This is advantageous over known configurations of sensorsbecause it reduces the possibility of solid material becoming trappedbetween opposing sensors.

Preferably, at least one sensor is capable of acting either as a sourceof an electric field or as a detector of an electric field. Providingeach of the sensors with this capability allows the distribution ofdetectors and sensors to be optimised for any required measurement.

Preferably, at least one of the sensors is located on a dielectricmounting. Where the dielectric mounting forms a casing in one surface ofwhich a sensor is located, an electric field obtained by applying avoltage to that sensor will retain its general shape in the event thatthe permittivity of the media surrounding the casing is increased.

Preferably, at least one of the sensors is separated by dielectricmaterial from an electrical connection to another of the sensors. Thisallows some sensors to be used as detectors and some to be used assources of electric fields.

The sensors may control for example a device for injecting flow controlchemical additives, or a flow choke device. Such a flow choke device maybe located upstream from or downstream from a T-junction defining oneinlet and two outlets, the flow choke being controlled to controlseparation processes occurring at the junction Thus an inlet flow to forexample an inclined plate separator can be pre-conditioned by divertingat least a proportion of unwanted components away from the inclinedseparator at a T-junction located upstream of that separator.

The invention also provides a system for monitoring the flow of a mixedphase fluid through a vessel the vessel defining surfaces adjacent towhich the fluid flows, wherein sensors are embedded in the saidsurfaces, each sensor providing an output representative of acharacteristic of the flow adjacent the surface within which it isembedded, and the sensors being distributed within the said surfacessuch that the location relative to the vessel of a boundary betweenphases of a flow can be determined from a comparison between the sensoroutputs.

The invention also provides a method of calibrating an array of sensorsconfigured to monitor the location of a boundary between layers offluid, the method comprising obtaining a series of measurements from twoof the sensors while the boundary between the layers of fluid is at aseries of locations between central areas of the sensors. Once the arrayof sensors has been calibrated for a given fluid or fluids, the positionof the boundary between the layers of the fluid or fluids can be veryaccurately determined.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described by way ofexample, with reference to the accompanying drawings, in which;

FIG. 1 is schematic representation of a sensor array capable ofmonitoring the separation of an oil-water mixture;

FIG. 2 is a schematic representation of a vertical array of capacitanceand pressure sensors which can be used to monitor the position of aninterface between components of an oil-water mixture:

FIG. 3 is a schematic illustration of a separator vessel incorporatinginclined plate and weir plate separators;

FIGS. 4, 5 and 6 each show vertical axial sections and verticaltransverse sections through three alternative flow sensing assemblies;

FIG. 7 schematically illustrates the distribution of capacitance sensorcomponents in two adjacent plates of an inclined plate separator such asthat incorporated in the structure illustrated in FIG. 3;

FIG. 8 illustrates in further detail the disposition of the sensorarrangement shown in FIG. 7;

FIG. 9 illustrates the structure of FIG. 8 in greater detail;

FIG. 10 represents the disposition of two vertical arrays of sensors inthe weir plate separator of FIG. 3;

FIG. 11 illustrates a vertically extending structure of a typeincorporated in the arrangement of FIG. 3; and

FIG. 12 illustrates control arrangement for a T-junction separator stagewhich may be incorporated upstream of the separator assembly of FIG. 3.

FIG. 13 illustrates a specific configuration of a vertical array ofsensors of the type which is shown schematically in FIG. 2;

FIG. 14 illustrates an alternative configuration of a vertical array ofsensors of the type which is shown schematically in FIG. 2;

FIG. 15 is a cross-sectional view of an alternative configuration of avertical array of sensors of the type which is shown schematically inFIG. 2;

FIG. 16 illustrates a series of layers which together comprise avertical array of sensors of the type which is shown schematically inFIG. 2;

FIG. 17 illustrates schematically a cross-section of a series of layerswhich together comprise a vertical array of sensors of the type which isshown schematically in FIG. 2;

FIG. 17a illustrates schematically a more detailed cross-section of thelayers shown in FIG. 17;

FIG. 18 is a schematic view of a cross-section through a source and adetector located in hemispherical casing;

FIG. 19 is a representation of a source and a detector showing theelectric field present when a voltage is applied to the source;

FIG. 20 is a schematic view of a cross-section through the separatorvessel of FIG. 3, showing an array of capacitance sensors, and oil andwater held in the vessel in separated layers.

FIG. 21 illustrates a section of the array of capacitance sensors shownin FIG. 20.

FIG. 22 is a graph representing a number of capacitance profilesobtained using array comprising a series of electrodes whilst aninterface between two media was traversed across the separator vessel ofFIG. 20.

FIG. 23 is a graph representing data from FIG. 21 which data has beenmathematically transformed.

FIG. 24 is a graph representing a computer simulation corresponding tothe data shown in FIG. 23.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, the illustrated system comprises a rectangularsection pipe 1 within a upper section of which is embedded a sourceelectrode 2. Four detection electrodes 3 are embedded in axially spacedlower portions of the pipe 1. Signals detected at the electrodes 3 aredelivered to a capacitance meter 4 the output of which is applied to adata acquisition system 5. The output of the data acquisition system 5is applied to a computer 6 which produces an estimate of the position ofan interface 7 between oil and water components of the flow (representedby arrow 8) within the pipe 1.

It will be appreciated that the position of the interfaces 7 within thepipe will vary over time and the resulting changes in the dielectriccoefficient of the region between each electrode 3 and the sourceelectrode 2 will vary. Such variations enable an estimate to be made ofthe relative volumes of the different phases passing through the pipe 1.

FIG. 2 is schematic illustration of a sensor array which can be used tomonitor the location of an interface between different phases in arelatively large vessel across which the illustrated array extends. Theillustrated arrangement comprises a support 9 on which thermocouples 10are positioned to enable temperature compensation of sensor outputs tobe achieved. Mounted on the support 9 is a sub-assembly 11 supporting avertically spaced array of piezoelectric pressure transducers 12 and avertically spaced array of embedded capacitance sensors 13. If the arrayis immersed in a vessel containing two vertically separated layers offor example oil and water, outputs from the sensors enable an estimateto be made of the vertical position of the interface between thedifferent layers.

Referring now to FIG. 3, this schematically represents a separator foruse in the separation of water, oil and gas components from an oil wellproduction flow The separator comprises a containment vessel 14 havingan inlet 15, a gas outlet 16, a produced water outlet 17, and an oiloutlet 18. Pipe sensors for example of the type illustrated in FIG. 1are represented as component 19 on the inlet 15 and components 20, 21and 22 on the outlets 16, 17 and 18. A further outlet 23 is provided forthe discharge of accumulated solids, and inlets 24 are provided toenable the agitation of deposited solids. Solids will generally bedischarged periodically and therefore the dynamic control of the output23 and input 24 is not required. This feature of the operation of theseparator will not therefore be further described herein.

The separator vessel 14 houses an inclined plate separator 25, avertically extending weir plate separator 26, an oil/gas separator 27and three sensor arrays 28, 29 and 30. With the exception of theinstrumentation supported on the units, the separators 25, 26 and 27 areconventional. The sensor arrays 28, 29 and 30 are of the general typedescribed with reference to FIG. 2. The separator plates 25 supportthree vertical arrays 31 of capacitance sensors. The weir plateseparator 26 supports two vertical arrays of capacitance sensors 32, andangled plates in the separator 27 each support a capacitance sensor. Thevarious sensing assemblies are connected by signal processing units 33,34, 35 and 36 to a controller 38 which in rum is arranged to controlvalves 39, 40, 41 and 42 provided on the inlet and gas, oil and wateroutlets respectively. The valve 42 can also be controlled directly bythe signal processing unit 35 such that if the oil/water interface onthe upstream side of the weir plate 26 rises above a predetermined levelthe valve 42 is open to discharge water. Similarly, the valve 41 can bedirectly controlled by a level gauge 43 which discharges oil throughvalve 41 automatically if the oil/gas interface exceeds an upper sensinglevel 44 and closes the valve 41 automatically if the oil/gas interfacefalls below a sensing level 45.

The sensor arrays 31 within the plate separator 25 enable the interfacebetween any flow of emulsion separating water and oil to be accuratelylocated. The sensor arrays 28, 29, 30 and 32 make it possible to monitorthe efficiency of the separation process in the direction of flowthrough the separator. This information is delivered to the controller38 which then ensures the appropriate control of the inlet and outletvalves to maintain appropriate flow and pressure conditions to preventundesirable circumstances developing, for example a circumstance suchthat significant volumes of emulsion are present in the separatordownstream of the inclined plate separator 25.

FIG. 4 shows the detailed structure of one possible pipe sensor whichdiffers slightly from that of FIG. 1. In the arrangement of FIG. 4, fiverings each of eight plates 46 are disposed around the axis of a shortflanged pipe insert. Appropriate signals may be applied to the plates 46so as to derive the necessary capacitance measurements. In thearrangement of FIG. 5, which is similar to that shown in FIG. 1, asingle lower plate 47 is located opposite five upper plates 48. In theembodiment of FIG. 6 a rectangular section channel is defined havingsingle plates 49 in upper and lower walls.

FIG. 7 illustrates the sensor structure in two adjacent plates 50 of theinclined plate separator 25 of FIG. 3. One plate carries a continuousexcitation source conductor 51 which faces an array of sensor electrodes52. FIG. 8 is a front view of the plate 50 carrying the electrodes 52.Electrodes 52 are surrounded by a guard electrode 52 a to obtain auniform electrical field in front of the detecting electrodes 52,between source (not shown in FIG. 8) and, detecting electrodes 52.Assuming a flow of oil, oil-water emulsion and water in the direction ofarrow 53, the interface between the emulsion and oil may be located asindicated by broken line 54 and the interface between the emulsion andwater may be located as indicated by broken line 55. It will beappreciated that signals derived from sensors located adjacent the bodyof emulsion will be substantially different from signals derived fromelectrodes adjacent either the oil or water phases.

The controller 38 of FIG. 3 is set up to monitor changes in the locationof the interfaces 54 and 55 as shown in FIG. 8 so as to prevent asubstantial proportion of space between the inclined separator platesbecoming filled with emulsion.

FIG. 9 shows one structural assembly which can be used to achievesensing electrode arrays of the type described-with reference to FIG. 7and 8. Each of the plates comprises an electromagnetic shield 56 andeach of the spaced detection electrodes 52 is surrounded by a guardelectrode 57. The electrodes may be in the form of conductive areassupported by a printed circuit board and housed within an epoxy resininsert secured to the plates 50 by screws 38. The plates could be formedfrom GRP.

FIG. 10 shows two vertical arrays of sensing electrodes 32 which areprovided in the weir plate 26 of FIG. 3. Each array may be independentlymonitored so as to provide comparative outputs between any verticallyaligned pair and any vertically adjacent pair, thereby enabling thedetection of any operational fault which might compromise the responseof the system to the oil/water interface approaching either the top ofthe weir or the outlet 17.

FIG. 11 illustrates in greater detail the structure of a sensor array ofthe type illustrated generally in FIG. 2 and used to define the sensorarrays 28, 29 and 30 in FIG. 3. The illustrated assembly comprises anelongate support 59 incorporating a vertical array of pressure ports 60each coupled to a piezoelectric pressure sensor (not shown). The support59 also supports an array of detection electrodes 61 surrounded by guardelectrode 62. A source electrode 63 is also mounted on the support 59spaced from the detection electrodes 61 such that the space between theelectrodes is filled by the fluid within the separator. One possiblelocation for an oil-emulsion interface is indicated by plane 64, and onepossible location for an emulsion-water interface is indicated by plane65. It will be appreciated that monitoring the outputs of the pressureand capacitance sensing transducers will enable the location of theinterfaces 64 and 65 to be accurately determined and thereby enable theappropriate control of the overall process to ensure that the interfaces64 and 65 are maintained within acceptable limits.

Referring now to FIG. 12, this shows a T-junction initial separatorwhich may be connected to the separator illustrated in FIG. 3 upstreamof the inlet 15. An inlet 66 is coupled through a valve 67 to aT-junction 68 that is coupled by a valve 69 to an outlet 70 and iscoupled directly to an outlet 71. Flow sensors 72, 73 and 74 for exampleof the type describe with reference to FIG. 4 are located on the inletto and the outlets from the T-junction 68. Outputs from those sensorsare processed in a signal processing unit 75 which controls the valves67 and 69.

It is known that supplying an oil/water mix to a T-junction as describedin which the two outlets are directed in vertically opposite directionsresults in some separation of the two components, water tending to flowvertically downwards and oil tending to flow vertically upwards. Thepresent invention enables this known effect to be optimised bycontrolling the flow to and back pressure within the T-junction 68 byappropriate modulation of the control applied to the valve 67 and 69.

This document does not contain detailed formula linking inputs to thesignal processing units and consequential valve control outputs.Detailed control algorithm will be required which will differ betweendifferent applications. To further explain the underlying designphilosophy however, relevant conditions which might apply in a threephase separator such as that illustrated in FIG. 3 are discussed below.

The inlet flow sensor 19 provides information on the relative quantitiesof the various phases flowing at any one time into the separator.Control actions might be applicable if for example large slugs of waterenter the vessel.

The vertically extending sensor arrays 28, 29 and 30 are distributedalong the direction of flow through the separator, one (28) beingbetween the inlet and the plate separator and enabling the efficiency ofthe flow distribution system to be monitored. If the flow distributionsystem becomes partially blocked by solids then the mass flux across theseparator vessel would change and this could adversely affect theseparation process in the rest of the unit. It would be possible todetect the development of such conditions from the upstream array 28.The two downstream arrays 29 and 30 make it possible to monitor theposition and depth of any heterogeneous layers such as emulsions betweenoil and water, and foam between gas and oil phases. These heterogeneouslayers are sometimes transitory and build up locally in the vesselparticularly towards the weir plate 26. If an array of sensors isembedded in the weir plate itself as shown by electrodes 32 in FIG. 3,his gives additional information as to the position of various phaseboundaries at the downstream end of the separation system. The outputsfrom the two vertical arrays 29 and 30 and the weir plate 26 enableoptimised control of the water outlet valve 42. Signals from the weirplate sensor electrodes 32 would be particularly useful to produce alarmsignals should the heterogeneous liquid-liquid or indeed the producedwater layer itself rise to be near the top of the weir plate. If thiscondition developed then entrainment of water in the separated oil wouldincrease rapidly. The instrumentation downstream of the plate separator25 would enable such conditions to be avoided.

Within the separator plate assembly 25, the three vertically alignedarrays of electrodes 31 make it possible to obtain an indication of themass flux distribution entering the plate system. It would also indicateif solids began to build up in the vessel upstream of the plate assemblyand within the plate assembly itself. It will be appreciated that ifsolids start to build up between the separator plates separationefficiency will be rapidly degraded. The present invention provides realtime measurements initiative of solids accumulation in the system.

With in the inclined plates separator, the axial separation of the threearrays of sensor electrodes 31 makes it possible to monitor theseparation process in the axial direction. Two conditions which coulddevelop and which would give rise to a loss of separation efficiency arechoking, that is local build-up of one of the phases, and instability,that is instability of the interface between water and oil in theindividual channels defined between adjacent plates. Both conditionsgive rise to rapid entrainment of the discontinuous phase and a fall inoverall separation performance. These conditions can be monitored withthe sensor electrode arrays as shown. By analysing the output of all thevarious sensor arrays, a control strategy can be developed to maintainoperation within a pre-defined desirable envelope.

The incorporation of sensors in the otherwise conventional separator 27which is provided on the gas outlet to eliminate mist, enables anybuild-up in liquid within the separator to be detected. If liquid buildsup then the probability of mist entrainment in the gas outlet increases.Entrainment of liquid in the gas leaving the vessel can produce seriousprocessing and safety conditions in downstream processes.

Although the controller 38 as described with reference to FIG. 3 is onlyused to control inlet and outlet valves, it will be appreciated that thecontroller may also be used to control the injection of chemicals toenhance phase separation, for example, by inhibiting the formation ofemulsions and foams.

The sensor arrangement shown in FIG. 2, and as 28, 29 and 30 in FIG. 3,can be replaced by an alternative arrangement shown in FIG. 13. A seriesof sensors in this case are electrodes 76 for measuring capacitance.Contrary to the conventional arrangement of sources spaced apart fromparallel detectors which allow fluid to flow between them, theelectrodes 76 are arranged on a single surface 77. This avoids thepossibility of solid matter becoming trapped between the electrodes 76.Each electrode 76 can be used as either a source (of an electric field)or a detector (of the electric field) by appropriate switchingcircuitry, and capacitance is measured between pairs of electrodes(probably adjacent electrodes, but not necessarily).

An alternative configuration of sensor array is shown in FIG. 14. Theelectrodes of this array comprise a series of rings 78 disposed atregular intervals along a rod 79. One possible mode of operation of thearray is illustrated schematically, wherein each of the rings 78labelled ‘s’ acts as a source and the ring 78 labelled ‘det’ acts as adetector.

FIG. 15 illustrates a further alternative configuration of sensor array.A ‘V’ shape is cut into a rod 80, and a source 81 is positioned in oneface and a series of detectors 82 is spaced along the other face of the‘V’. This sensor array is conventional in that it comprises a singlesource 81 which is not capable of acting as a detector and a series ofdetectors 82 spaced away from the source 81, which detectors are notcapable of acting as a source. This array configuration is advantageousover known conventional configurations in that solid material isunlikely to become trapped between the faces of the ‘V’. A typical anglesubtended by the faces of the ‘V’ is 120 degrees. Screens 83 are locatedbehind the sources 81 and the detectors 82.

An array of electrodes which has minimal undesired “cross-talk” effectbetween electrodes and connections to electrodes is illustrated in FIG.16. The way comprises six layers of printed circuit board (PCB) 84-89which are bonded on top of one another as shown in FIG. 17. The dark(solid) regions in FIG. 16 correspond to conducting material.

A first of the layers 84 of the array comprises a series of electrodes90 surrounded by a guard electrode 91. A second layer 85 comprises ashielding conductor 92, with electrical connections 93 leading to signalcarrying connections in further layers of the array.

A third layer 86 contains signal carrying connections 94 and a shieldingconductor (94 b) which prevents “cross-talk” between connections 94 inthe third layer 86. In the present example, layer 86 allows connectionsfrom 10 electrodes 90 of layer 84, via electrical connections 93 in thesecond layer 85 to “soldering points” 95 at an upper end of the thirdlayer 86.

A fourth layer 87 is essentially the same as layer 85. It shields thethird layer 86 from a fifth layer 88 but provides electrical connectionsbetween electrodes 90 and signal carrying connections 94 in the fifthlayer 88.

The fifth layer 88 is conceptually similar to the third layer 86 andprovides connections from eight remaining electrodes 90 to the“soldering points” 95 at an upper end of the layer 88. The signalcarrying connections 94 are shielded from one another by conductor 94 b.The sixth layer 89 serves as a shield between layer 88 and an externalenvironment.

In general, for capacitance measuring electrodes, when the surface areaof a connection to a given electrode becomes comparable to the surfacearea of that electrode, the measured capacitance will be influenced bythe connection. When a conventional ‘dipstick’ arrangement of electrodesis immersed in a medium of given permittivity, each electrode shouldmeasure the same capacitance. However, the effect of the surface areasof the electrode connections is such that those electrodes near a lowerend of the dipstick will measure a greater capacitance than those nearan upper end of the dipstick. Tie array of electrodes illustrated inFIGS. 16 and 17 avoids this problem by shielding the electrodes 90 fromthe signal carrying connections 94, and the signal carrying connections94 from one another. Efficient shielding is provided by the conductingplates 92 of the second 85 and fourth 87 layers of the array, whichprohibit cross-talk between the layers 84, 86 and 88.

To obtain a desired performance of the sensors it is necessary to“hardwire” the following components: (i) guard electrode 91, (ii)shielding conductor 92 in the second layer 85, (iii) shielding conductor94 b in the third layer 86, (iv) shielding conductor 92 in the fourthlayer 87, (v) shielding conductor 94 b in the fifth layer 88, (vi)shielding conductor comprising the sixth layer 89, in as many places aspossible. In the discussed example this has been achieved by the processof “through-hole plating”, while the sensors were manufactured usingprinted circuit board (PCB) technology. The connections are omitted inFIG. 16.

Connections between electrodes 90, connections 93 in layers 85 and 87and signal carrying connections 94 in layers 86 and 88 were similarlymade using “through-hole plating” technique.

The technique of shielding between individual layers in the “sandwich”described with reference to FIG. 16, a well as between individual signalcarrying connections in layers 86 and 88 of FIG. 16 is advantageousbecause it allows both:

avoidance of cross-talk between signals from individual electrodes; aswell as

the use of each electrode as a source or detector as described withreference to FIG. 13. If the connections to the electrodes were notshielded then “cross-talk” between the sources and the detectors wouldbe imposed on the measured signal, reducing the accuracy of measurementsignificantly.

The array of FIG. 16 is just one example of many possible ways ofshielding the signal carrying connectors. A section through ageneralised array of electrodes is shown in FIG. 17. An uppermostsurface and a lowermost surface of the array are formed from laminate97. A series of electrodes 98 are connected, through a series of shields99 to a series of connectors 100. The number of electrodes, shields andconnectors may be tailored to any required purpose.

A generalised array of the type shown in FIG. 17 is shown in more detailin FIG. 17a. The cross-section of FIG. 17a is across a width of anarray. Laminate 97 again forms the upper and lower surfaces of thearray. A detection electrode 98 is provided with a guard electrode 98 aat either side. A series of shielding layers 99 separate the detectionelectrode from a series of signal carrying connections 100. Each signalcarrying connection 100 is isolated from its neighbours by separationconductors 100 a.

FIG. 18 shows a source 101 and a detector 102 located on parallel facesof two parts 103 of a circular rod (middle section removed). A shield104 is located behind both the source 101 and the detector 102. Thearrangement of detector/shield in the bottom half of FIG. 18 will beprobably the generalised array from FIG. 16 and 17. The supports 103illustrated are casings which may be constructed from metal or fromdielectric material. When dielectric material is used, an electric fieldobtained from the source 101 is closer to being a uniform field thanthat obtained when a metal support 103 is used.

FIGS. 19a and 19 b illustrate a source arid detector, of the formillustrated in FIG. 18, held in a metal casing. When the source anddetector are located in air (FIG. 19a), the electric field obtained isclose to parallel, but when they are immersed in water (FIG. 19b) thefield degenerates significantly. Thus, as the permittivity of the mediasurrounding the detector and source is increased, the quality ofelectric field obtained is degraded. In contrast, when the source anddetector are held in a dielectric casing (FIGS. 19c and 19 d), theelectric field obtained is close to parallel both in air (FIG. 19c) orin water (FIG. 19d). Thus, the dielectric casing is advantageous becauseit provides an electric field which is close to parallel when thedetector and source are located in media of a range of permittivities.

FIG. 20 shows a cross-section of the separator vessel of FIG. 3,containing water oil arid air in three layers. An inclined plateseparator 105 is provided with an array 106 of capacitance sensors. Asection of the array 105 is illustrated in more detail in FIG. 21. Anupper electrode of the array is a source 107, and the lower electrodesare a series of four detectors 108.

The array may be calibrated to allow detection of the position of theoil/water interface to an accuracy better than the height of oneelectrode 108. The calibration procedure comprises:

displacement of the interface, in a vertical direction, in small steps(at least a few steps per electrode height) along the entire length ofthe array 105;

storing the differences in readings from all adjacent pairs ofelectrodes (eg.: . . . C_(k−2)-C_(k−1), C_(k−1)-C_(k), C_(k)-C_(k+1),C_(k+1)-C_(k+2), . . .)

The above differences are zero (assuming that the electrodes areidentical and far from “fringe” effects which occur close to ends of thearray) if the pairs of adjacent electrodes 108 are immersed in the samemedium (oil or water). However, when the two adjacent electrodes 108 arein different media, the difference between their readings reaches amaximum. All “intermediate” interface positions produce “intermediate”values C_(k)-C_(k+1), and these a used to identify the position of theinterface with a high resolution.

FIG. 22 shows a number of capacitance profiles obtained for an arraycomprising a series of eighteen electrodes, while an interface level wastraversed across the vessel.

FIG. 23 shows data from FIG. 22, read from two electrodes of the array.The data has been “transformed” into co-ordinates (C₁₀-C₉) versus oillayer thickness (or an arbitrary position of the interface). This“spike-like” function is unique for a given media permittivity. Bylooking at differences between neighbouring electrodes the position ofthe interface can be established to an accuracy greater than the heightof one electrode.

FIG. 24 shows a computer simulation where permittivity of distilledwater (ε_(r)=80) was replaced by different values (50, 20 and 5.6). Thecharacter of the calibration curve remains unchanged but the maximumvaries. This implies the need for separate calibrations for allcombinations of media for which the sensor array is to be used.

What is claimed is:
 1. A sensor array for detecting the distribution offluids having different electrical characteristics, comprising amultilayer structure including a first layer which defines an array ofspaced apart sensor electrodes, a second layer separated from the firstlayer by dielectric material and defining a conductive screen, and athird layer separated from the second layer by dielectric material anddefining a series of spaced apart elongate connections, wherein thesecond layer is sandwiched between the first and third layers, eachsensor being connected to a respective connection by a respectiveconductive path extending through an opening in the conductive screendefined by the second layer.
 2. A sensor array according to claim 1,wherein the conductive paths are defined by through-hole platingstructures formed through apertures in the dielectric material.
 3. Asensor array according to claims 1, wherein the first layer whichdefines the array of sensor electrodes is covered by dielectricmaterial.
 4. A sensor array for detecting the distribution of fluidshaving different electrical characteristics, comprising a multilayerstructure including a first layer which defines an array of spaced apartsensor electrodes, a second layer separated from the first layer bydielectric material and defining a conductive screen, a third layerseparated from the second layer by dielectric material and defining aseries of spaced apart elongate connections, and a fourth layerseparated from the third layer by dielectric material and defining aconductive screen such that the third layer is sandwiched between theconductive screens defined by the second and fourth layers each sensorbeing connected to a respective connection by a respective conductivepath extending through an opening in the conductive screen defined bythe second layer.
 5. A sensor array according to claim 4, comprising afifth layer separated from the third layer by dielectric material anddefining a conductive screen, and a sixth layer separated from thefourth and fifth layers by dielectric material and defining a furtherseries of spaced apart elongate connections, selected sensors beingconnected to respective further connections by respective conductivepaths extending through openings in the conductive screens defined bythe second and fifth layers.
 6. A sensor array according to claim 4,wherein the layers are supported on dielectric boards which define thedielectric material separating adjacent layers.
 7. A sensor array fordetecting the distribution of fluids having different electricalcharacteristics, comprising a multilayer structure including a firstlayer which defines an array of spaced apart sensor electrodes, a secondlayer separated from the first layer by dielectric material and defininga conductive screen, a third layer separated from the second layer bydielectric material and defining a series of spaced apart elongateconnections, each sensor being connected to a respective connection by arespective conductive path extending through an opening in theconductive screen defined by the second layer, wherein the third layerdefining elongate connections incorporates a conductive screen extendingbetween each adjacent pair of elongate connections.
 8. A sensor arrayfor detecting the distribution of fluids having different electricalcharacteristics, comprising a multilayer structure including a firstlayer which defines an array of spaced apart sensor electrodes, a secondlayer separated from the first layer by dielectric material and defininga conductive screen, a third layer separated from the second layer bydielectric material and defining a series of spaced apart elongateconnections, each sensor being connected to a respective connection by arespective conductive path extending through an opening in theconductive screen defined by the second layer, wherein the first layerincorporates a conductive screen spaced from the sensor electrodes anddefining a series of openings, each sensor electrode being locatedwithin a respective opening.